Magnesium base alloys



Aug. 20, 1963 E. F. EMLr-:Y

MAGNESIUM BASE ALLOYS Filed 001;. 9, 1961 FIG. 3.

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lUnite States 3,101,269 MAGNESUM BASE ALLOYS l Edward Frederick Emley, Manchester, England, assigner --to Magnesium Elektron LimitedManchester, England Filed Oct. 9, 1951, Ser. No. 143,839

Y Claims priority, application Great Britain Oct. 18, 1960 16 Claims. (Cl. 75-168) This invention `relates to magnesium zirconium alloys v of the type disclosed in British Patent No. 511,137 and in tain up to about 4% lof zirconium in solutiongalthough `the maximum solubility of zirconium inthe Yliquid alloy is only about 0.7%` at normal casting temperatures. ,When such alloys are-worked, the zirconium-rich cores become drawn lout into zirconium-rich striations. Thesel .Worked alloys show good resistance to loss of room temperature properties Ifollowing annealing or hot working, and this phenomenonhas been attributed to the effect Vof these zirconium-rich striations in` restricting graingrowth. 1

Where such Wrought alloys are `to be used at high tem-v peratures for long periods, for example in nuclear reactors, there is the possibility that the striations may gradu- ,ally disappear `owing to diffusion yof zirconium throughout the microstructure with accompanying undesirable .graintgrowth There is further the Ipossibility, for example in the case Iof the binary alloy, that the zirconium striations will become the -seat of hydride precipitation arising through slow pick-up of hydrogen derived from moisture in the surrounding atmosphere. these effects, such -alloys cannot be regarded as indeni-tely; stable `for use at temperatures where these processes ,can occur.

In order to produce'such worked alloys in lthe stable condition', it has been proposed to heat treat the worked alloys, for example extrusions of the binary alloy, at temperatures of labout 600 C., in order to homogenise the .alloy by diffusion of the zirconium. We have found this process to be very slow and accompanied by considerable Lgrain growth, whilst the resultant alloys are still liable to .precipitation of hydride. If such extrusions are deliberately heated in hydrogen, or a moist atmosphere, precipitation-free zones are formed near the grain boundaries,

' and -the final produc-t shows poor creepy ductility attemperatures arou-nd 2004.50 C. f

In order to produce tine grained wrought zirconium alloys of high stability, we have subjected a cast or extruded billet to a precipitation treatment, so as to precipitate -t-he zirconium as hydride (with possibly some nitride) rby heat treatment in a suitable atmosphere containing 4available hydrogen,'i.e. hydrogen, ammonia, moisture or atet As a result of v the' .original billet, the alloys possess excellent creep ductility.

Table I illustrates the high ductility obtainable at 200 lC. in accordance with the present invention when the hydriding step is carried out before rather than after extrusion. Thus extrusions C and D, which were hydrided 31,101,269 Patented Aug'. 20, 1953 ice before extrusion, show elongations of 178 and 108% in Extrusion i Stress, Time of llonga-A No. Condition t.s.i. v Fracture tion (hrs.) (percent) Extruded only 1.0 336 58 do 1.5 50 47 Hydrided and extruded-.. 0.5 480 178 dO 1.0 43 108 Homogenised, extruded l. 5 60 8 and hydrided. do 2.0 9 14 Homogenised, extruded... 1.5 168 50 Homogenised, hydrided, 1.0 740 42 extruded. do 1. 5 48 41 Homogenisa-tion treatment: 20 hrs. at 625 C. in boric acid. Hydriding treatment: V16A hrs. at 550 C. in moist hydrogen. Extrusion temperature: 400 C. approx.V

The hydrogen treatment may be carried out at a temperature not less than 300 C., and preferably, not greater than 600 C., the time of treatment depending mainly on the maximum thickness of section of the billet or otherv workpiece. Thus at 550 C. in atmospheres such as dry hydrogen, Wet hydrogen, wet CO2, or wet argon or steam, heat treatment times ot -the'order of 6 hrs. per in. of section thickness are generally appropriate.

At lower temperatures, eg. 400 C., the hydriding process occurs less readily, but satis-factory hydriding in rea-sonable times can be yachieved if the components are rst subjected to a shot-blasting treatment. This elect probably works in two ways: by contaminating the surface with iron, so providing electrolytic couples `to assist decomposition of water vapour, and also by roughening the surface layers. As an alternative to shot-'blasting the surface may be roughened or activated in other ways, c g. by treating it with salt solution or a Asolution containing an iron or other heavy metal salt. The lower hydriding, temperatures produce finer precipitates than are obtained by hydriding at 550 C. and are consequently preferred where a stiler product is desired.

The quantity of hydrogen which can readily be introduced into the alloy, c g. at 550 C., is 20G-250 cc. per

g. of Zr content, and corresponds roughly with the for- Y mula ZrH2. We have found that under some circumstances, e.g. with shot-blasted specimens at 450 C. using steam or wet hydrogen, it is possible to introduce considerably higher proportions of hydrogen into the alloy, e.g. 400 cc. per g. of Zr content. The quantity of hydrogen should 'be at least 80 cc. per gnam of zirconium. Also, the quantity of hydrogen in the alloy should beat lease 0.002 percent by weight of Ithe alloy, i.e. at least 22 cc. per 100 grams of the alloy. If desired,` the hydrogen treatment may be carried out under pressure, eg.

at several atmospheres.

tOther elements which may be included in the alloy are 'hydride Exclusion of rare earth metals and thorium is,

however, desirable where the final alloy is to be welded, since their hydrides can decompose, giving rise to gas bubbles in the weld.

We have found that the concentration of zirconium in fthe cores in the middle of the grains is higher inthe "show excellent weldability, the resulting welds being quite free from gas porosity.

According to one aspect of the present invention, therefore, a ne .grained wrought magnesium alloy, which is metallurgically stable at high temperatures and Ipossesses good creep ductility, has the following composition:

Zr lOl-1%,

H At least 0.002% (i.e. atleast 22 cc. per 100 g. alloy),

Mg The remainder.

the hydrogen content being at least 80 cc. per gram zirconium content, and the microstructure of the alloy being characterised by the absence of precipitation-free bands near the grain boundaries.

The maximum zirconium content preferred is 0.7% and the preferred hydrogen range is 22-3 50 cc. per 1l00 g. alloy, there being at least 100 cc. hydrogen per gram of zirconium content. The manganese contributes to the stiffness of the alloy at high temperatures, but adds to the capture cross section of the alloy for thermal neutrons. Accordingly, the manganese content is preferably restricted to not more than 0.2% and may usefully be 0.1-0.2% in cases where the presence of some manganese is desired. lSince manganese and zirconium are to some extent incompatible in magnesium, the sum of the manganese and zirconium contents should not exceed 0.8% and the product of these contents expressed in percentage should not exceed 0.1.

A minimum zirconium content of 0.25% is also desinable, with a hydrogen content of 50-250'cc. per 100 g. alloy, but with at least 100 cc. hydrogen per gram zirconium content. For best results, We use a zirconium content of 0.4 to 0.7% with 125 to 175 cc. hydrogen per 100 g. alloy, a particularly suitable composition being OAS-0.65% Zr -with about 140 cc. hydrogen per 100 gram alloy.

Beryllium may be incorporated in Ithe alloy in amounts up to |aboutr0.01% with a view to improving the oxidation resistance of the alloy (both in the molxen state and in the solid state at high temperatures) and also to minimising grain growth during annealing subsequent to Working.

We have -found that with a full zirconium content, c g. 0.6-0.7%, it is only possible to dissolve about 0.0015 lberyllium in the liquid alloy, the balance of the beryllium content being present in the form of insoluble beryllium rich intermetallic particles. This amount of dissolved beryllium is suflicient to produce detectable reduction in tendency to oxidise, but is far from suicient to ,achieve the marked suppression of the burning tendency which can be obtained with beryllium in magnesium and its alloys which do not contain zirconium. We have further found that if the zirconium content of a magnesium-zirconiumalloy is decreased, the amount of beryllium which can 'be taken in solution is increased. Thus, if the zirconium content is reduced to the level of 0.05 to 0.1% the beryllium solubility is raised to about 0.005%, i.e. to

substantially the same level as in pure magnesium. In consequence, such alloys with lowered zirconium content and containing up to 0.005% dissolved beryllium show very advantageous suppression of the tendency to oxidise.

For some purposes, it may be desirable to homogenise the billet or other workpiece before subjecting it to the hyclrding step. The zirconium hydride in the nal product will then be present in substantially uni-form dispersion or diffuse bands, in contrast with material which has been hydrided and extruded without 'an additional homogenisation; in the latter case the zirconium hydride precipitation is present in narrow, sharply defined bands separated lby regions of greater width which are substantially free from zirconium precipitation. Such homogenisation treatment is only worth carrying out where the zirconium .content ofthe alloy exceeds about 0.4%, otherwise the zirconium distribution is more uniform throughout-the Vmicrostructure and Without the relatively small intense concentrations of dissolved zirconium (the well-knoWn"-cores) characteristic of alloys with high zirconium content. Alloys prepared by extrusion of homogenised and hydrided billet possess high resistance to grain growth with creep ductility similar to that in the normal extruded state. v

According toV a further aspect of the invention, therefore, a tine `grained wrought zirconium all-oy is produced containing at least 0.4% zirconium together with at least 22 cc. hydrogen per 100 g. alloy (with manganese, rare earth metals, thorium, and/or beryllium if desired .in the above specified ranges) and with microstructure charaeterised by the presence of precipitation lin substantially uniform `dispersion or diffuse bands.

For certain purposes, e.g. fuel element cans operating in the high temperature regions of gas cooled graphite moderated reactors, the reduction in creep ductility associated with the formation of precipitation-free zones in 'wrought products which have been hydrided after working may not be serious. Accordingly, alloys which have been made by homogenisation followed by extrusion and -hydriding treatments may be valuable. The increased stiffness which can be @obtained when manufacturing extiusions by this route is also shown in the above table. Lofw temperature hydride treatment, eg. 450 C., is de sinabie to minimise grain growth and surface oxidation on the linal product.

According to another aspect =of the present invention, therefore, la wrought zirconium alloy exhibiting good resistance to grain growth and good creep resistance at a high temperature contains at least 0.4% zirconium with at least 22 cc. hydrogen per 100 g. alloy, the alloy containing manganesre and/or beryllium if desired in the above specilied ranges but not rare earth metals and thorium, and a microstructure characterised by the presence of precipitation in substantially uniform dispersion or diffuse bands, together with pnecipitation-ree zones near the grain boundaries.

If desired, a line grained wrought alloy can be prepared by homogenisation prior to extrusion. In this way, we have made an alloy yshowing greater stiffness than normal extruded alloys but with equally good creep ductility (see Table I, extrusion G) 4and good resistance to grain growth. Such a product is not, however, metallurgically stable at high temperatures in moist atmospheres, since it is possible `for Athe zirconium to precipitate as hydride.

For lthe homogenisation treatment the 'furnace atmosphere should be substantially free from hydrogen or moisture and accordingly the hydrogen content will not exceed 20 cc. per grams of the alloy.

According to 'a funtlrer aspect of the present invention, therefore, a fine grained wrought magnesium alloy with enhanced creep resistance and good grain growth stability contains 0.4 to 0.7 zirconiumin substantially uniform solution or in diffuse bands, the alloy containing mangenese and/or Yberyllium if desired a-s before. The distribution of the zirconium in `solution can be readily seen by subjecting the alloy to a 'hydniding treatment, eg. at 500 C. -in moist hydrogen, and .subsequently etching a polished microsection in 2% nital.

The preferred homogenisation temperature is at least 625C., and Ithe preferred time atleast 24 hrs. Alternatively, -a longer period such as 100 hrs. may be used at a lower temperature, e.g. 600 C. lllt is desirable duning homogenisation heat treatments to exclude sources of hydrogen, such :as moisture :in the atmosphere, until the treatment is complete. Accordingly, heat treatnnt may 'be carried out in dry augon, with a fused boric acid coating, or in a sealed container.

We have found that good results can be obtained in accordance with the present invention when starting with either aV cast billet or a lbillet cut from extruded bar.

The excellent resistance to grain growth provided by alloys in accordance with the present invention is illusrtrated for two of its 'preferred embodiments -in Table II which relates to grain growth after pre-straining 5%% at 250 C., followed by heat treatment yfor 100 hrs. at

' 450 C. of an alloy fof composition Mg0.'6% Zr.

Homogenisation treatment: hrs. at 625 C.

Hydriding treatment: 16 hrs. at 550 C.

The eiect-s achieved with and without the use of this invention are illustrated in the accompanying drawing which diagrammatically shows the microstructure `of extruded'alloys consisting [of 0.5% zirconium, remainder magnesium, at a magniication of 150.

FIGURE 1 shows the Ialloy which has been hydri'ded after extrusion.

y The structure shows (aU-coarse grain, (b) precipitation B in very distinct bands, and (c) precipitation-free zones C yalongside the grain boundaries.

The bands of precipitate consist vof zirconium hydride and :aire formed by spreading out of the zirconium-rich solid solution corings when the cast vstruct-ure is worked.y

The precipitation-free zones alongside the grain boundaries show that the alloy has been lhydrided subsequently to working.

FIGURE 2 shows the alloy when hydrided before extrusion.v

The microstructure shows (a) line grain, (b) precipitation D in very distinct bands and (c) no precipitation-free zones alongside the grain boundaries.

FIGURE 3 shows the alloy when homogenised, then hydrided and finally extruded.

The microstructure shows (a) line grain, (b) general-ly uniform precipitation E but with indications of diffuse bandsand (c) no precipitation-free zones near :grain boundaries.

Uniormity of the distribution of precipitate shows that the .alloy has been homogenis'ed before hydride treatment, and the absence of precipitation-free zones alongside the grain boundaries shows that Iit has been hydrided before extrusion. p

FIGURE 4 shows the alloy when homogenized initially but hydrided after extrusion.

The microstructure shows (a) coarse grain, (b) general-lyuniform precipitation with indications of `diffuse bands, and (c) precipitation-tree zones alongside the grain boundaries. i

The generally uniform precipitation indicates that the alloy has been homogenized before hydride precipitation, Iand the precipitation-free zones show that the hydriding treatment was applied after extrusion.

6 I claim: 1. A wrought magnesium alloy consisting ofthe following parts by weight:

' the rare earth metals `and lthorium together not exceeding r Zr percent-- 4.0%, the hydrogen content being at least cc. per gra-rn zirconium content, and the microstructure of the lalloy being characterised'in that the distribution of the precipitate does not change in 4the neighborhood of the grain boundaries.

2. A wrought magnesium alloy as claimed -in claim 1, with the following parts by weight:

H 22-350 cc. per g. alloy.

Mg Remainder.

the hydrogen content being at least 100 cc. per gram zirconium content.

3. A wrought magnesium lalloy as claimed in claim l with the following parts by weight:

H 50-250 cc. per 100 g. alloy.

Mg Remainder.

the hydrogen content being at least 100 cc. per gram zirconium content. f 3.

im "l 4. A wrought magnesium alloy as claimed in cl with the following parts by weight:

H 12S-175 cc. per

100 g. alloy. Mg Remainder.

5. A wrought magnesium zirconium alloy, having a composition las claimed in claim 1, in which the zirconium content is at least 0.4% land the microstructure is characterised 4by precipitation in substantially uniform dispersion kor in diffuse bands lying in the direction of metal llow.

6. A wrought magnesium zirconium yalloy having a composition as claimed in claim 1, excluding rare earth metals land thorium in which the zirconium content is at least 0.4% and rthe microstructure is characterised by precipitation in substantially uniform Vdispersion or in diffuse bands lying in the direction of metal ow together with precipitation-free zones near .-the grain boundaries.

7. A wrought Valloy having the following parts by weight:

Zr 0.40.7%. v

H Not exceeding 20 oc.

per 100 g. of alloy. 1

'Mg Remain-der.

, and manganese .is less than 0.2%.

9. A pro-cess for producing la wrought mag-nesiumalloy which consists in subjecting la cast or extruded billet or other workpiece of the following parts by weight:

7 Mn do -0.3 RE. do 0-3.0 'Ih do @-3.0 Be do 0-0.0l Mg Remainder the rare earth metals'and thorium together not exceeding 4%, to a hydriding treatment, said hydriding treatment being elected in an atmosphere containing hydrogen in a form capable of :being absorbed into the alloy, for at least half tan hour at la temperature of at least 300 C., so las to introduce at least 0.002% hydrogen into the alloy and subjecting the workpiece to plastic deformation. 10. A process as claimed in claim 9 in which the billet or Iother workpiece "has the following parts by weight:

Zr pereent 0.1-0.7 Mn \do 0-0.2 Be do 0-0.01 Mig Remainder 11. A process for producing a tine-grained wrought magnesium alloy, which consists in effecting solution heat treatment of a workpiece of magnesium alloy of the following parts by weight:

Zr percent 0.4-0.7 Mn do 0-0.2 Be do O-0.01 Mg Remainder said heat treatment being eiected at a temperature exceeding 575 C. for at least 4 hrs. and subjecting the workpiece to plastic deformation.

12. A process as claimed in claim 9, in which a hydride treatment is carried out at a temperature of 40G-600 C. for at least 5 hrs. per in. of maximum section thickness.

13. A process as `claimed in claim 9, in which the workpiece is shot-blasted prior to hydride treatment.

Zr percent 0.4-0.7 Mn do 0-0.2 Be 'do 0-0.01

Mg Remainder l5 said heat treatment being effected at a temperature exceeding 575 C. `for at least 4 hrs., subjecting the workpiece to a hydriding treatment in an atmosphere containing hydrogen in a form capable of being absorbed into the alloy at a temperature of atleast 300 C. and then subjectmg the workpiece to plastic deformation.

References Cited in the file of this patent UNITED STATES PATENTS Jessup July 22, 1952 FOREIGN PATENTS Canada May 27, 1958 OTHER REFERENCES Ball et al., Further Progress in the Development of Mg-Zr Alloys to Give Good Creep land Fatigue Properties Between 500 and 650 F. Journal of Metals, trans. AIME, July 1953. 

1. A WROUGHT MAGNESIUM ALLOY CONSISTING OF THE FOLLOWING PARTS BY WEIGHT: ZR 0.1-1%. H AT LEAST 22 CC. PER 100 G. ALLOY. MN 0-0.3%. R.E 0-3.0% TH 0-3.0%. BE 0-0.01% MG REMAINDER. THE RARE EARTH METALS AND THORIUM TOGETHER NOT EXCEEDING 4.0%, THE HYDROGEN CONTENT BEING AT LEAST 80 CC. PER GRAM ZIRCONIUM CONTENT, AND THE MICROSTRUCTURE OF THE ALLOY 